The invention relates to a system and method for heating and cooling compressor inlet air supplied to a gas turbine for the purpose of maximizing power output and efficiency across the load and ambient range.
The combined cycle power plant in its simplest form is comprised of a gas turbine, a steam turbine, a generator, and a heat recovery steam generator (HRSG), with the gas turbine and steam turbine coupled to the single generator, in tandem on a single shaft. Multi-shaft arrangements having one or more gas turbine generators and a common steam turbine generator have been utilized. The thermal efficiency of combined cycle power plants is determined by the performance of the gas turbine in combination with its heat recovery bottoming cycle.
Gas turbines have two characteristics that can become obstacles to achieving maximum output and efficiency across the operating range.
First, gas turbines are constant volume flow machines. This gives them the natural characteristic of lower air mass flow and power output on warm days, when air is less dense, than on cooler days, when the air is more dense. In this regard, air density increases with decreasing temperature and results in higher mass flow through the gas turbine. The gas turbine output increases with increased air mass flow. It is thus often found desirable to increase air mass flow on warm days, typically by reducing inlet air temperature, to increase output.
Second, working fluid is supplied to the cycle by an air compressor which has aeromechanical operating limits that constrain operating compression ratio as a function of corrected speed, corrected air flow, and variable guide vane setting. Turbine inlet volume flow, and hence compressor pressure ratio, increases on cooler days since supply air mass flow increases with decreasing compressor inlet temperature while firing temperature is maintained (for maximum efficiency). Accordingly, the compressor operating limit can become an issue on colder days. Compressor operating limits are also more easily impinged in applications that burn dilute fuel, which also increases turbine volume flow and hence compressor pressure ratio. This situation may occur as a consequence of fuel composition or gasification of a low grade fuel which may produce fuel gas only one fifth as rich as natural gas.
It should also be noted that gas turbine compressors include one or more rows of variable guide vanes which can be modulated to reduce compressor inlet air flow. Although this may seem to offer a means of limiting compressor pressure ratio, and thus avoiding the compressor operating limit, the operating limit is itself a function of the guide vane setting such that the limit becomes more severe as air flow is reduced in this manner.